feat: treeshake `module.exports` property write by h-a-n-a · Pull Request #8659 · rolldown/rolldown

h-a-n-a · 2026-03-12T16:33:01Z

Extended the side effect detector to identify module.exports.property = ... patterns as PureCjs operations.

This adds on top of the existing exports.xxx support.

// side-effect free if xxx is a static property and yyy is side-effect free
module.exports.xxx = yyy

This PR previously detects the side-effect of module.exports as a whole. But there's some issue with tree-shaking. Will take a look at it once I have a thorough knowledge of this.

h-a-n-a · 2026-03-12T16:33:14Z

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Boshen · 2026-03-13T05:24:02Z

Side effect detector is being moved to Oxc.

h-a-n-a · 2026-03-13T07:46:09Z

Side effect detector is being moved to Oxc.

Does it include CJS side-effect detection?

h-a-n-a · 2026-03-13T08:09:25Z

Let's wait for the migration in #8624 complete and come back to this.

codspeed-hq · 2026-04-01T07:05:03Z

Merging this PR will not alter performance

✅ 4 untouched benchmarks
⏩ 10 skipped benchmarks¹

_{Comparing 03-13-feat_detect_module.exports_related_assignment_as_side-effect_free (fc7b86a) with 04-01-feat_support_inlineconst_for_cjs_exports_accessed_through_module.exports (f1e6e0d)}

10 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

Copilot

Pull request overview

Extends CommonJS export analysis to treat module.exports.<staticProp> = ... as a side-effect-free (“PureCjs”) operation, enabling additional tree-shaking opportunities similar to existing exports.<prop> handling.

Changes:

Updated side-effect detection to classify module.exports.<staticProp> = ... as PureCjs.
Enhanced AST scanning to recognize module.exports.<prop> writes as named CommonJS exports and track them via new EcmaModuleAstUsage flags.
Updated integration snapshots to reflect new tree-shaking/output behavior.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
crates/rolldown/src/ast_scanner/side_effect_detector/mod.rs	Detects `module.exports.<staticProp>` assignments as `PureCjs` and adds unit tests.
crates/rolldown/src/ast_scanner/impl_visit.rs	Tracks `module.exports.<prop>` writes as CommonJS named exports; introduces new AST usage flags and normalization logic.
crates/rolldown_common/src/ecmascript/ecma_view.rs	Adds `EcmaModuleAstUsage` flags to distinguish `module.exports` property writes vs other `module` usages.
crates/rolldown/tests/rolldown/topics/exports/entry_cjs_named_default/artifacts.snap	Snapshot updated for new generated output/export wiring.
crates/rolldown/tests/esbuild/default/warn_common_js_exports_in_esm_convert/artifacts.snap	Snapshot updated showing `module.exports.<prop>` write can now be tree-shaken from emitted output.

Copilot · 2026-04-01T07:41:24Z

crates/rolldown/src/ast_scanner/impl_visit.rs

+                self.result.ast_usage.insert(EcmaModuleAstUsage::ModuleExportsPropWriteRef);
+              }
+              _ => self.result.ast_usage.insert(EcmaModuleAstUsage::ModuleExportsNonPropWriteRef),


In process_identifier_ref_by_scope for the global module identifier, CommonJsAstType::EsModuleFlag currently falls into the default arm and sets ModuleExportsNonPropWriteRef. That means module.exports.__esModule = true is treated like a non-property write and will later clear AllStaticExportPropertyAccess, which defeats the intent of allowing module.exports.<static> writes to remain treeshakeable. Consider treating EsModuleFlag as a static module.exports property write for the purpose of setting ModuleExportsPropWriteRef (without necessarily incrementing cjs_named_exports_usage).

Suggested change

self.result.ast_usage.insert(EcmaModuleAstUsage::ModuleExportsPropWriteRef);

}

_ => self.result.ast_usage.insert(EcmaModuleAstUsage::ModuleExportsNonPropWriteRef),

self.result

.ast_usage

.insert(EcmaModuleAstUsage::ModuleExportsPropWriteRef);

}

Some(CommonJsAstType::EsModuleFlag) => {

// Treat `module.exports.__esModule = true` as a static property write so it

// doesn't clear `AllStaticExportPropertyAccess`, but don't track it as a

// named export in `cjs_named_exports_usage`.

self

.result

.ast_usage

.insert(EcmaModuleAstUsage::ModuleExportsPropWriteRef);

}

_ => self

.result

.ast_usage

.insert(EcmaModuleAstUsage::ModuleExportsNonPropWriteRef),

Copilot · 2026-04-01T07:41:25Z

crates/rolldown/src/ast_scanner/impl_visit.rs

+              // `module.exports.test = ...` — create facade symbol like `exports.test = ...`
+              if let Some((span, export_name)) = member_expr.static_property_info() {
+                let exported_symbol =
+                  self.result.symbol_ref_db.create_facade_root_symbol_ref(export_name);
+
+                self.declare_link_only_symbol_ref(exported_symbol.symbol);
+
+                if let Some(value) = self.extract_constant_value_from_expr(Some(&node.right)) {
+                  self
+                    .add_constant_symbol(exported_symbol.symbol, ConstExportMeta::new(value, true));
+                }
+


The new module.exports.<prop> = ... handling records a constant export based solely on node.right. For compound assignments like module.exports.foo += 1, node.right can be a literal but the exported value is not that literal, so this would incorrectly mark foo as a constant and may cause invalid inlining. Gate the constant extraction on the assignment operator being a simple = assignment (and consider applying the same guard to the existing exports.<prop> branch too).

Copilot · 2026-04-01T07:41:25Z

crates/rolldown/src/ast_scanner/impl_visit.rs

+    //
+    // Access apart from module.exports.xxx or exports.xxx
+    // will be considered as non-static property access
+    if self.result.ast_usage.contains(EcmaModuleAstUsage::ModuleExportsNonPropWriteRef)
+      || (!self.result.ast_usage.contains(EcmaModuleAstUsage::ModuleExportsPropWriteRef)
+        && !self.result.ast_usage.contains(EcmaModuleAstUsage::ExportsRef))
    {
      self.result.ast_usage.remove(EcmaModuleAstUsage::AllStaticExportPropertyAccess);


The comment above the AllStaticExportPropertyAccess normalization says “Access apart from module.exports.xxx or exports.xxx will be considered as non-static property access”, but the actual logic is driven by ModuleExports{Prop,NonProp}WriteRef and ExportsRef and will also clear the flag for cases like module.exports.foo reads and module.exports = ... assignments. Updating the comment to reflect that this is specifically about static property writes (and that other module/exports uses bail out) would make the intent clearer.

github-actions bot assigned h-a-n-a Mar 12, 2026

h-a-n-a force-pushed the 03-13-feat_detect_module.exports_related_assignment_as_side-effect_free branch from d24aa33 to 4ce7eaf Compare March 12, 2026 16:39

hyf0 mentioned this pull request Mar 13, 2026

refactor(treeshake): migrate SideEffectDetector to Oxc's MayHaveSideEffects trait #8624

Merged

1 task

h-a-n-a force-pushed the 03-13-feat_detect_module.exports_related_assignment_as_side-effect_free branch from 539e74a to 2efe7b1 Compare March 13, 2026 03:37

h-a-n-a force-pushed the 03-13-feat_detect_module.exports_related_assignment_as_side-effect_free branch 3 times, most recently from a3b062f to a0be4d1 Compare March 28, 2026 15:08

h-a-n-a changed the title ~~feat: detect module.exports related assignment as side-effect free~~ feat: treeshake module.exports property write Mar 28, 2026

h-a-n-a changed the base branch from main to graphite-base/8659 April 1, 2026 06:50

h-a-n-a force-pushed the 03-13-feat_detect_module.exports_related_assignment_as_side-effect_free branch from a0be4d1 to 4f1d736 Compare April 1, 2026 06:50

h-a-n-a changed the base branch from graphite-base/8659 to 04-01-feat_support_inlineconst_for_cjs_exports_accessed_through_module.exports April 1, 2026 06:50

This was referenced Apr 1, 2026

feat: support inlineConst for CJS exports accessed through module.exports #8976

Open

fix: correct inlining based on module's def format and esModule flag #8975

Merged

h-a-n-a force-pushed the 03-13-feat_detect_module.exports_related_assignment_as_side-effect_free branch from 4f1d736 to b3330ef Compare April 1, 2026 06:58

h-a-n-a marked this pull request as ready for review April 1, 2026 06:59

h-a-n-a requested a review from IWANABETHATGUY April 1, 2026 06:59

h-a-n-a requested a review from Copilot April 1, 2026 07:33

Copilot started reviewing on behalf of h-a-n-a April 1, 2026 07:34 View session

Copilot AI reviewed Apr 1, 2026

View reviewed changes

feat: detect module.exports related assignment as side-effect free

fc7b86a

h-a-n-a force-pushed the 03-13-feat_detect_module.exports_related_assignment_as_side-effect_free branch from b3330ef to fc7b86a Compare April 1, 2026 07:58

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feat: treeshake `module.exports` property write#8659

feat: treeshake `module.exports` property write#8659
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03-13-feat_detect_module.exports_related_assignment_as_side-effect_free

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